In external drum imaging systems, a movable optical carriage is commonly used to displace an image exposing or recording source in a slow scan direction while a cylindrical drum supporting recording media on an external surface thereof is rotated with respect to the image exposing source. The drum rotation causes the recording media to advance past the exposing source along a direction which is substantially perpendicular to the slow scan direction. The recording media is therefore advanced past the exposing source by the rotating drum in a fast scan direction.
An image exposing source may include an optical system for scanning one or more exposing or recording beams. Each recording beam may be separately modulated according to a digital information signal representing data corresponding to the image to be recorded.
The recording media to be imaged by an external drum imaging system is commonly supplied in discrete, flexible sheets and may comprise a plurality of plates, hereinafter collectively referred to as “plates” or “printing plates.” Each printing plate may comprise one or more layers supported by a support substrate, which for many printing plates is a plano-graphic aluminum sheet. Other layers may include one or more image recording (i.e., “imageable”) layers such as a photosensitive, radiation sensitive, or thermally sensitive layer, or other chemically or physically alterable layers. Printing plates which are supported by a polyester support are also known and can be used in the present invention. Printing plates are available in a wide variety of sizes, typically ranging, e.g., from 9″×12″, or smaller, to 58″×80″, or larger.
A cassette is often used to supply a plurality of unexposed printing plates to an external drum imaging system. The printing plates are normally supplied in stacks of ten to one hundred, depending upon plate thickness, and are stored in a cassette. Interleaf sheets, commonly referred to as “slip sheets,” may be positioned between the printing plates to protect the emulsion side of the printing plates, which is extremely vulnerable to physical damage, such as scratches, which could render a printing plate unusable for subsequent printing. When interleaf sheets are not used, great care must be taken to avoid emulsion damage as each printing plate is separated from the stack, fed from the cassette into the external drum imaging system, and mounted onto the external drum. Unfortunately, preventing such damage as the printing plates are unloaded and fed from a cassette to an external drum has proven to be a very difficult and expensive task in currently available external drum imaging systems, especially when larger (e.g., 45″ wide) printing plates are used.